Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss. Optical networks often employ wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM) to increase transmission capacity. In WDM and DWDM networks, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is based on the number of wavelengths, or channels, in each fiber and the bandwidth, or size of the channels. In WDM, DWDM and other optical networks, arrayed waveguide gratings (AWGs), interleavers, and/or fibergratings (FGs) can be used to add and drop traffic at network nodes and to multiplex and demultiplex traffic at network nodes. To enable reconfiguring the wavelength paths to be dropped or added at different nodes, network nodes having optical switches can be provided, known as all optical ROADM (Reconfigurable Optical Add Drop Multiplexer) nodes.
Currently the optical transport network is deployed in metro and core networks by using such nodes with a high capacity optical transmission interface at 10 Gbps in metro networks and 40 Gbps optical interface for core networks. The use of an all optical ROADM provides the network with the flexibility needed to adapt the wavelength paths to the network changes without manual intervention. It also allows a decrease of node power consumption and footprint due to the all optical transparency of the channels transporting transit traffic that do not need any OEO conversion and consequent electronic processing.
All optical ROADM nodes are typically based on optical devices like WSS (Wavelength Selective Switch) or PLC (Planar Lightwave Circuit) switches [see ref 1] to achieve the needed network flexibility, high capacity and resilience.
All Optical ROADMs are not able to provide traffic grooming and sub-wavelength switching, hence additional equipments like digital cross-connects can be used.
In the last few years an alternative concept has been introduced for the implementation of ROADM nodes, namely the ‘Digital ROADM’ concept [see ref 2].
In a digital ROADM, the incoming multiplexed optical signals are demultiplexed, O-E converted and processed by OTN (optical transport network) framers in the transponder units. Grooming and switching of the different signals is performed by the electrical switch while OTN processing and E-O conversion is accomplished before the optical multiplexing at the node output.
In the Digital ROADM node the two functions of optical transport and traffic grooming are integrated in single OEO switching equipment in which all the incoming traffic is OEO converted and subsequently processed independently if it is pass-through or locally added/dropped [see ref 3].
It has been appreciated that DWDM does not provide its own overhead for management nor protection schemes to recover from equipment failures. DWDM involves more network elements than earlier point to point optical links. Such elements, such as—optical amplifiers, multiplexers, and demultiplexers—and dispersion compensation units, can bring reliability concerns and warrant monitoring. As a result, the G.709 Optical Transport Network, or OTN standard was generated by the International Telecommunication Union Telecommunication Standardization (ITU-T) to provide management functionality for DWDM optical networks. OTN involves adding a frame of overhead information, (also called a digital wrapper), some to the front of the signal as a header, and some such as FEC (Forward Error Correction) as a trailer appended to the rear. The FEC can extend optical span distances by reducing bit error rates (BERs). U.S. Pat. No. 7,469,103 shows an example of a node providing transparent add drop multiplexing, with a framer to provide the digital wrapper. The framer is arranged to process all outgoing and all incoming signals as soon as they have been wavelength demultiplexed and converted to electrical form.